The uplink transmission of a user terminal can be received in general at several base-stations (multi-point reception), although typically a single base station controls the uplink transmission of that terminal. Traditionally, the intended receiver for that transmission is this controlling base station.
For example in UMTS a technique denoted as macro-diversity is employed. Inter-NodeB macro-diversity (soft handover) means that the signal is received and detected at more than one base station and the related information from all involved base stations is exploited at a central node (in the case of UMTS, at the radio network controller (RNC)). With intra-NodeB macro-diversity (softer handover) information received in different cells, which are all served by the same base station, is combined and exploited internally at that particular base station.
An emerging technique is to utilize received signals from different base stations (cooperative base stations). Similar to inter-NodeB macro-diversity this decreases the block error rate and increases the spectral efficiency of transmissions. However, the inter-NodeB macro-diversity requires UE involvement whereas cooperative base stations are transparent to the terminal. In 3GPP LTE-Advanced BS cooperation is being discussed as inter-eNB coordinated multi-point transmission and reception (CoMP), as e.g. specified in 3GPP TR 36.814, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Further Advancements for E-UTRA Physical Layer Aspects, V0.3.1, January 2009.
According to this document, intra-eNB CoMP is also being discussed for LTE-Advanced. There, cooperation is based on multiple antennas, which logically belong to the same eNB or even the same cell. These antennas may be geographically separated. With intra-eNB CoMP the signals from different antennas do not need to be exchanged between base stations. However, the exchange of information between the antennas and the base station introduces delays, too.
Since base stations or the cell controllers of multiple cells within a base station are typically physically separated, the information exchange between them introduces a certain delay. Depending on the deployment and the connectivity among cell-controllers the delay may be e.g. in an order of a few μs to several ms.
State-of-the-art wireless access systems typically employ an Automatic Repeat Request mechanism, e.g. a Hybrid Automatic Repeat Request (HARQ) mechanism, at the link layer to increase the spectral efficiency of the system. Such a mechanism uses feedback messages sent from the data receiver to the data sender in order to trigger retransmissions if the previous transmission failed. Incremental Redundancy and Chase Combining are two typical strategies to combat such transmission errors.
In order to minimize the transmission cost and the delays of the ARQ respectively HARQ feedback, state-of-the-art ARQ/HARQ mechanisms employ a fixed timing relation between the transmission of the signal (from the sender to the receiver) and the transmission of the ARQ/HARQ feedback (from the receiver to the sender). The fixed timing replaces other references between the transmitted signal and the corresponding feedback such as a sequence number, which require significantly more transmission resource.
As explained above, state-of-the-art radio ARQ/HARQ protocols typically assume a fixed and constant round trip time (RTT) of the ARQ/HARQ protocol. An example for this is the HARQ protocol in LTE as defined in the MAC protocol specification (3GPP TS E-UTRA MAC Specification 36.321). For LTE FDD the HARQ RTT is 8 ms, see the time between the 1st transport block and the redundancy version in FIG. 1. For LTE TDD the HARQ RTT varies depending on TDD configurations and on other timing aspects, but is also predetermined.
It is also specified for LTE that the HARQ feedback needs to be sent after a certain number of TTIs (or equivalently subframes). For example for LTE FDD the feedback needs to be sent 4 TTIs after the transmission, see FIG. 1. This means there are tight timing constraints for the uplink HARQ protocol operation.
If the concept of cooperative base stations shall be realized in the LTE, HARQ timing puts very tight delay requirements on the transmission and processing delays between physically separated receivers (base stations or geographically separated antennas/cells). Connections with very low delay (e.g. dedicated optical fiber) is typically very costly while inexpensive solutions do not fulfill the HARQ timing requirements.
FIGS. 2 and 3 show examples of a message sequence chart of cooperating BSs in the context of LTE FDD.
In FIG. 2, the serving base station (BS) sends a request for support to the supporting base station (BS), subsequent to which reception (Rx) of the transmission takes place in both the serving BS and the supporting BS. After having received the transmission, the supporting BS processes the signal and responds to the serving BS. Especially the transfer of the response message (information resp. IQ data transfer) is expected to be time consuming. Having received the response (information transfer), the serving BS needs to combine its own reception and the reception of the supporting BS (processing step). HARQ feedback is then transmitted by the serving BS after 4TTIs. Depending on the actual implementation, a potentially late request and additional processing prior to the request (shown in dashed lines) consumes time as well.
According to FIG. 3, a main receiver (main RX) and a cooperating receiver (comp RX) cooperate in decoding transmitted data. After uplink (UL) scheduling in TTI n−1, transmission is granted and the request for cooperation is sent to the cooperating receiver in TTI n. In TTI n+4, data transmission takes place, subsequent to which processing of the received signal (e.g. A/D conversion, FFT etc.) has to take place in both receivers (TTI n+5). Then, the IQ data is transferred according to the cooperation procedure, and processing, decoding etc. takes place. In TTI n+8 (i.e. 4 TTIs after data transmission as specified according to the HARQ protocol), the feedback, namely acknowledgement (ACK) or negative acknowledgement (NACK) is sent.
Therefore, it is in most cases not possible to use the concept of cooperating base stations, since the delay constraints can not be met with the typically employed transmission infrastructure between base stations, e.g. microwave links, Ethernet, DSL, E1/T1, etc. Note that, regardless of the number of TTIs mentioned in the examples herein, which may be different for different transmission systems, in any case time constraints are introduced by the mentioned feedback process.